The basic operation and structure of land mobile dispatch radio systems is known. Land mobile dispatch radio systems typically comprise one or more radio communication units and one or more repeaters that transceive information via communications channels. These communications channels typically reside on narrow bandwidths of 12.5 kHz, 25 kHz, or 30 kHz spaced channels. Further, within these narrow spaced channels, transmission and reception occurs within an even more narrow passband, such as 15 kHz. Such land mobile radio dispatch systems typically employ frequency modulation to convey speech signals.
In order to understand the embodiment described herein, it will be helpful to the reader to first understand the difference between a dispatch radio environment and other radio environments, such as a radio telephony operating environment. Unlike radio telephony systems, where a user establishes contact with another party by inputting a unique ID code (such as a telephone number) that specifically identifies the party to be called, dispatch services do not require the originating party to enter any particular ID code. Instead, dispatch radios are pre-programmed to automatically identify their transmissions as being intended for reception by members of a particularly pre-identified fleet. The user need only actuate a push-to-talk (PTT) switch on the radio and begin speaking. The radio, in response to enablement of the PTT switch, will automatically signal that a communication for the members of the previously defined group is being made, and the radio dispatch system will accommodate that circumstance.
Video transmission systems are also known. Audio and video transmissions are typically broadcast one-way to one or more receiving units. Typical channels are spaced by 6MHz. Within the typical channel, the standard method is to convey the intensity of the moving electron beam using amplitude modulation (the electron beam intensity corresponds to the darkness of the picture at a moving spot on the video image). The moving spot traverses the image in rows across the screen from top to bottom, at approximately 30 or 60 times a second. The rate of row advancement is typically 15 kHz. As a net effect, this AM modulated luminance signal typically occupies several MHz of bandwidth within a 6 MHz spaced channel. The video channel also usually carries audio on an FM carrier within the channel.
Therefore, standard video transmissions are not compatible with standard land mobile dispatch communications channels due to increased spectrum requirements.
Various video bandwidth compression techniques are known. The best known techniques involve taking a single frame of a full motion video, and transmitting it using what is commonly called slow scan technology. Similar methods are employed for the digitization and compressed transmission of scanned printed material, commonly called FAX. Both slow scan and FAX have been demonstrated over land mobile dispatch radio channels using video cameras and FAX machines connected to existing land mobile dispatch radio units through adapters.
Using land mobile dispatch radio to serve video needs is desirable. To date, however, using a land mobile dispatch radio as a video modem typically dedicates the radio to this service, thus precluding the radio's primary function; the provision of voice connectivity. Further, external video equipment coupled to the radio usually hinders the user's mobility.
Therefore, a need exists for a method of integrating total communications capabilities including audio, video imaging, and imaging display capabilities into a single land mobile dispatch radio system that will not substantially detract from the portability or utility of the communications unit, while maintaining compatibility with existing land mobile dispatch radio channels.